1. Introduction
The automatic segmentation of the pulmonary vasculature is an essential preprocessing step for many algorithms that analyze thoracic CT scans. An accurate segmentation can be used to exclude the vessels from further analysis, for instance to improve nodule detection algorithms. Also, the vessel tree segmentation itself can be used for the detection of pulmonary embolisms and for quantification of hypertension in which the diameter of the arteries provide an indicator for cardiovascular risk.